International Journal of Application or Innovation in Engineering & Management (IJAIEM)
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Volume 3, Issue 2, February 2014
ISSN 2319 - 4847
Physical properties of Ga0.03Zn0.97O thin film
prepared by spray pyrolysis: Dependence on
annealing temperature
Salam Amir Yousif1, Sana Juma Ali2
1,2
Department of Physics, College of Education, the University of Mustansiriyah, Baghdad, Iraq
ABSTRACT
Gallium doped zinc oxide (GZO) thin film has been deposited by spray pyrolysis technique onto glass substrates. The Effect of
annealing temperature on the structural, morphological and optical properties of Ga0.03 Zn0.097 O thin film has been investigated.
All the films are polycrystalline with a preferential c-axis (002) orientation. It is shown that the full width at half maximum
(FWHM) value decreases from 0.4472 to 0.2717 0 with increasing annealing temperature indicating the improvement of the
crystalline quality of Ga0.03Zn0.097O film. The surface morphology changed with the varying annealing temperatures. It is
observed that, as the annealing temperature increased, the diffusion of atoms on the surface of glass substrate was enhanced,
resulting in a flat or less rugged surface with (RMS) roughness 0.929 nm for the sample annealed at 450oC. Therefore it is
concluded that the sufficient surface diffusion leads to the improvement of crystalline quality which accompanies the modification
modification of the surface morphology. The as-deposited film was less transparent in the visible and IR regions than other
annealed films and the transparency of the films improved as the annealing temperature increased up to 450oC and then it
decreases for further increasing in annealing temperature at 500oC.
1. INTRODUCTION
ZnO is a II-VI semiconductor with a large band gap (approx. 3.4 eV) at room temperature and has large exciton binding
energy (~60 meV) [1]. Wide-band gap oxide semiconductor for optical devices, liquid crystal displays, and solar cells
have attracted much attention. ZnO has been a promising material for many different applications such as gas sensors [2],
transparent electrodes [3], piezoelectric devices [4], varistors [5], and surface acoustic wave devices [6]. ZnO thin film is
fabricated by several methods: sputtering [7], [8], reactive plasma deposition (RPD) [9], [10], pulsed laser deposition
(PLD) [11], metal organic vapor phase epitaxy (MOVPE) [12] and spray pyrolysis [13], [14].
While synthesizing transparent conducting oxide (TCO) thin films, it is a common practice to introduce impurities such
as In, Al, Ga, N, P, As and F [15], within ZnO thin films in order to enhance the optoelectronic properties. All the
transparent conductive doped ZnO films provide excellent UV shielding due to the absorption edge on the short
wavelength side (λ~300nm)[16]. Beside, high reflectance in the IR region can make this a good candidate for application
as the coating for IR reflecting mirror or heat reflector, as well as in other field of technology [17], [18]. All these
applications require films with good uniformity, high electrical conductivity and high electron concentration
simultaneously with high transmittance to visible light and high reflectance to near-IR light [19], [20]. From many
reported investigations of the doped effect of impurity on ZnO, Ga doped seems to be the most successful and promising
due to its advantages, such as the similar atomic radius compared to Zn, which could result in only small lattice
deformation even for the case of high Ga concentrations [21], [22]. Also, Ga is less reactive and more resistive to
oxidation compared to Al [23], [24]. Therefore, Ga doping can produce higher electron concentration than Al doping,
which may be more useful for improving the reflective behavior of transparent conductive ZnO films according to Drude
theory.
In this study, we fabricated and characterized Ga0.03 Zn0.097O single-layer film as the first step of the application
mentioned above. Then, effect of annealing temperature on the several fundamental properties such as structural,
morphological and optical properties of the obtained film has been investigated.
2. EXPERIMENTAL
The Ga0.03Zn0.097O thin film was deposited by spray pyrolysis technique under ambient atmosphere. Two kinds of
aqueous solutions, Zinc acetate Zn(CH3COO)2.2H2O and Gallium nitrate Ga(NO3)2 ,were chosen as the sources of zinc
and gallium respectively. The concentration of the solutions was kept at 0.1M and a few drops of HC1 were added to the
solution to increase the solubility of the compounds. The Ga0.03Zn0.097O film was prepared onto ultrasonically cleaned
glass substrate held at a substrate temperature of 350o C. The substrate temperature was maintained constant using
temperature controller with an accuracy of ±10oC. The solution flow rate was fixed at 6ml/min. The samples were
annealed under ambient atmosphere at 400, 450, 500 oC for 1hour.
The crystal structure of Ga0.03Zn0.097O thin films was investigated by X-ray diffraction (XRD) using SHIMADZU X-Ray
diffractometer system (XRD - 6000). The surface morphology of Ga0.03Zn0.097O thin film has been examined by using
atomic force microscopy (AFM, scanning probe microscope).The optical measurements of Ga0.03Zn0.097O thin film is
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Volume 3, Issue 2, February 2014
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calculated from the transmittance and absorbance spectrum at normal incidence over the range (300 – 900 nm), by using
UV-VIS spectrophotometer type (SHIMADZU) (UV-1600/1700 series).
3. RESULTS AND DISCUSSION
3.1. STRUCTURAL PROPERTIES
The crystal orientation and crystallinity of the film was determined by X-ray diffraction. Figure 1 shows the XRD spectra
of Ga0.03 Zn0.097O thin film as-deposited and annealed at 400, 450, 500 oC under ambient atmosphere. At as-deposited
and all annealing temperature the films are polycrystalline with a preferential c-axis (002) orientation along with the
other (101), (100) reflections that corresponding to the hexagonal wurtzite structure of ZnO. This is because the similar
radii between the Zn 2+ ions and Ga3+ ions enable the substitution of Ga for Zn without any significant change in the lattice
constants of Ga0.03Zn0.097O compared to those in ZnO. The observed d-values are presented in Table 1, and compared
with the standard ones from the (JCPDS) data file (36-1451) [25]. This fact also indicates no change of the wurtzite
structure, well-consist with other reports [26]-[30].
It is shown that the full width at half maximum (FWHM) value decreases from 0.4472 to 0.27170 with increasing
annealing temperature indicating the improvement of the crystalline quality of Ga0.03Zn0.097O film. The crystalline
quality of the films depends strongly on the annealing temperature due to the enhanced surface diffusion that comes with
increasing annealing temperature, which is consist with the report [31].
Figure 1 XRD pattern of Ga0.03Zn0.097O thin film for different annealing temperature
Table1: XRD parameters of as-deposited and annealed Ga0.03Zn0.097O thin film
asJCPDS for
400
450
500
Annealing Temperature
deposited
ZnO
2θ(deg.)
31.739
31.742
31.751
31.753
31.77
(100)
d(Ǻ)
2.8169
2.8167
2.8159
2.8157
2.816
Int.
58
56
40
40
57
2θ(deg.)
34.437
34.455
34.482
34.485
34.422
(hkl)
(002)
d(Ǻ)
2.6021
2.6008
2.5989
2.5986
2.602
Int.
100
100
100
100
44
2θ(deg.)
36.214
36.214
36.241
36.242
36.253
(101)
d(Ǻ)
2.4784
2.4784
2.4766
2.4766
2.476
Int.
88
84
67
65
100
The unit cell parameter (a, c) and the ratio of (c/a) belong to the (002) plane as a preferred orientation of Ga0.03Zn0.097 O
thin films deposited on glass substrate have been calculated using equation (1) [32]. The calculated values of lattice
parameter (a, c) and (c/a) are shown in Figures 2,3 and listed in Table 2, found to be in a good agreement with the
standard (JCPDS) values [25].
1
4h 2  hk  k 2 l 2
(1)

 2
2
d
3a 2
C
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Table 2: Lattice constants of Ga0.03Zn0.097O thin film
a (Å)
c (Å)
annealing temperature
as-deposited (350)
3.25273
5.20434
400
3.25247
5.20172
450
3.25157
5.19786
500
3.25137
5.19728
(JCPDS)
3.24982
5.20661
c/a
1.5999
1.5993
1.5985
1.5984
1.6021
Figure 2 a-axis lengths vs. annealing temperature
Figure 3 c-axis length vs. annealing temperature
The crystallite sizes of as-deposited and annealed Ga0.03Zn0.097O thin films prepared on glass substrate are calculated
using the full width at half maximum (FWHM) of (002) peak employing Scherer Formula equation (2)[33]. It can be
noted that the value of crystallite size increases with increasing annealing temperature, because the crystallite size is
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inversely proportional with the full width at half maximum. The values of crystallite size of Ga0.03 Zn0.097O thin films are
shown in Figure 4, and listed in Table 3.
0.9 
D
(2)
 cos 
Where λ is the wavelength of X-ray used (1.5406 Å), β is the full-width at half maximum (FWHM) of the peak which has
maximum intensity and θ is the Bragg angle.
Figure 4 Crystallite size vs. annealing temperature
The texture coefficient (Tc) evaluated from equation (3) [33] is used to describe the preferred orientation of
Ga0.03Zn0.097 O thin films. It is clear that the deviation of the texture coefficient from unity implies the preferred
orientation of growth. The larger the texture coefficient deviates from unity, the higher will be the preferred orientation of
Ga0.03Zn0.097 O thin films. The variation of texture coefficient estimated along (002) direction with annealing temperature
of Ga0.03Zn0.097O film is shown in Figure 5. It was found that the value of texture coefficient increases with increasing
annealing temperature as shown in Table 3.
I(hkl)/I o (hkl)
TC (hkl) 
(3)
1
N r  I( hkl) /I o (hkl)
Figure 5 Texture coefficients vs. annealing temperature
The micro strain depends directly on the lattice constant (c) and its value related to the shift from the ASTM standard
value. The values of micro strain of as-deposited and annealed Ga0.03Zn0.097O films are listed in Table 3. This strain can
be calculated from the following formula [34].
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ε  
C ASTM  C XRD
C ASTM
(4)
X100%
The dislocation density ( ) , defined as the length of dislocation lines per unit volume, has been estimated using the
following equation [35], [36].
1
 2
(5)
D
To derive the film stress (σfilm) parallel to the film surface, the following formula is used, which is valid for a hexagonal
lattice [37].
σ film
2
2c13
 c33 (c11  c12 ) c film  cbulk


2c13
cbulk
(6)
For the elastic constants cij, the following values for single crystal ZnO are used, C11 = 208.8 GPa, C33 = 213.8 GPa, C12 =
119.7 GPa, C13 = 104.2 GPa, Where: cfilm and cbulk are the lattice constants (c) of ZnO film and bulk (or powder).
Table 3: Structural properties of Ga0.03 Zn0.097O film for different annealing temperature
Annealing
temperature
Crystallite
Size
(nm)
Texture
Coefficient TC
Micro
Strain
ε%
Dislocation Density
(δ)
Flim
stress
(GPa)
as‫ـ‬deposited
350
18.6
(101)
(100)
(002)
0.043
2.890
0.1001
400
21.8
0.633
0.731
1.63
0.093
2.104
0.2165
450
28.2
0.615
0.719
1.66
0.168
1.257
0.3911
500
30.6
0.551
0.577
1.87
0.179
1.067
0.4167
3.2. MORPHOLOGY
Figure 6 shows the AFM images of Ga0.03Zn0.097O samples as-deposited and annealed at 400, 450, 500 oC . As shown in
the figure, the surface morphology changed with the varying annealing temperatures. For the as-deposited film the
surface morphology was rough and rugged, and the root mean square (RMS) roughness of this film was 1.29 nm. This is
attributed to the fact that the surface diffusion was insufficient due to the low deposition temperature [38].However, after
annealing the film at 400, 450, 500 oC under ambient atmosphere, the rough of the surface decreases with increasing
annealing temperature up to 450oC and then it is increased for 500o C. The root mean square and grain size of
Ga0.03Zn0.097 O as-deposited and annealed have been listed in Table 4.
It is observed that, as the annealing temperature increased; the diffusion of atoms on the surface of glass substrate was
enhanced, resulting in a flat or less rugged surface with (RMS) roughness 0.929 nm for the sample annealed at 450oC.
Therefore it is concluded that the sufficient surface diffusion leads to the improvement of crystalline quality which
accompanies the modification of the surface morphology.
Table 4: RMS roughness, Grain size and peak-peak height of Ga0.03Zn0.097O film
annealing temperature
RMS roughness (nm)
Grain size (nm)
Peak-Peak height
(nm)
as-deposited 350
1.29
134.19
12.1
400
1.23
104.61
8.74
450
0.929
78.78
4.4
500
1.77
86.62
8.43
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Figure 6 AFM images of Ga0.03Zn0.097 O thin film (a) as-deposited, (b) annealing at 400oC, (c) annealing at 450oC and
(d) annealing at 500oC
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3.3. OPTICAL PROPERTIES
Figure 7 shows the optical transmittance spectra taken at room temperature. The annealed Ga0.03Zn0.097O films were
highly transparent in the visible and IR regions from 400 to 900 nm and showed sharp absorption edge in the UV region.
For the as-deposited film the transmittance is low compare with those of annealed films and no sharp absorption edge
observed in this film. This indicates that the substrate temperature 350oC is not sufficient to arrange Ga and Zn ions sites
in the lattice. We suggest that the band gap shift are directly proportional to the Ga ions which are substituted for the Zn
ions in the film without changing the wurtzite structure, and at low deposition temperature, it is though that the Ga atoms
are not incorporated in the ZnO lattice and are partially located at defect sites such as interstitials in Ga0.03Zn0.097 O
lattice. Furthermore, the as-deposited film was less transparent in the visible and IR regions than other annealed films
and the transparency of the films improved as the annealing temperature increased up to 450o C and then it decreases for
further increasing in annealing temperature at 500oC. The decrease in the transmittance for the film annealed at 500oC
can be described to the decreasing Ga content as a result of the increasing of re-evaporation rate of Ga atoms from the
Ga0.03Zn0.097 O film [31].
Figure 7 Transmittance spectra of Ga0.03Zn0.097O film at different annealing temperature
The absorbance spectra of the Ga0.03Zn0.097O thin film deposited on glass substrate at as-deposited and different
annealing temperature that measured at room temperature is shown in Figure 8. In the high energy spectral range, where
the film is strongly absorbent, and the absorbance of Ga0.03 Zn0.097O films decrease with increasing of annealing
temperature from, 350℃ (as-deposited) to 450℃, then it goes on increases with increasing substrate temperature from
450℃ to 500℃.
Figure 8 Absorbance spectra of Ga0.03 Zn0.097O film at different annealing temperature
The variation of absorption coefficient against photon energy hγ for direct band-to-band transition has the form:
Αhγ=A(hγ-Eg)X where hγ is the photon energy, Eg is the band gap, A is the edge parameter. The value of x is (1/2) for
direct allowed transitions and (2) for indirect allowed transition. The films prepared in the present study have direct
allowed transitions. Hence plots have been made by taken (hγ) along x-axis and (αhγ)2 along y-axis as shown in Figure 9.
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2
Figure 9 Variation of (h ) vs. photon energy ( h ) of Ga0.03Zn0.097O thin film
The values of direct band gap are estimated from the extrapolated of the linear portion of the curve to zero absorption and
found to be (3.25, 3.32, 3.32, 3.3) eV for the as-deposited and annealed Ga0.03Zn0.097O film at 400,450,500oC
respectively, which are in agreement with the report [27] . From Figure 9 we can see that the bandgap of Ga0.03Zn0.097 O
thin film increases with increasing annealing temperature from (3.25 eV) at as-deposited 350℃ to (3.32 eV) at annealing
temperatures 400, 450 ℃ and, then it decreases with increasing annealing temperatures from (3.32 eV) to (3.3 eV) at
annealing temperatures 500℃.
4. CONCLUSION
Ga0.03Zn0.097 O thin films have been deposited successfully onto glass substrates at 350 oCby spray pyrolysis technique.
The full width at half maximum (FWHM) value decreases from 0.4472 to 0.27170 with increasing annealing temperature
indicating the improvement of the crystalline quality of Ga0.03Zn0.097O film. The crystalline quality of the films depends
strongly on the annealing temperature due to the enhanced surface diffusion that comes with increasing annealing
temperature.
The surface morphology changed with the varying annealing temperatures. The surface morphology of as-deposited film
was rough and rugged because the surface diffusion was insufficient due to the low deposition temperature. However,
after annealing the film, the rough of the surface decreases with increasing annealing temperature up to 450 oC and then
it is increased at 500 oC. As the annealing temperature increased, the diffusion of atoms on the surface of glass substrate
was enhanced, resulting in a flat or less rugged surface with (RMS) roughness 0.929 nm for the sample annealed at
450oC. Therefore it is concluded that the sufficient surface diffusion leads to the improvement of crystalline quality which
accompanies the modification of the surface morphology.
For the as-deposited film the transmittance is low compare with those of annealed films and no sharp absorption edge
observed in this film. This indicates that the substrate temperature 350 oC is not sufficient to arrange Ga and Zn ions sites
in the lattice. At low deposition temperature, it is though that the Ga atoms are not incorporated in the ZnO lattice and
are partially located at defect sites such as interstitials in Ga0.03Zn0.097O lattice. Furthermore, the as-deposited film was
less transparent in the visible and IR regions than other annealed films and the transparency of the films improved as the
annealing temperature increased up to 450 oC and then it decreases for further increasing in annealing temperature at
500 oC. The decrease in the transmittance for the film annealed a 500 oC can be described to the decreasing Ga content as
a result of the increasing of re-evaporation rate of Ga atoms from the film.
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